MEMS ribbon arrays may be operated as very fast analog optical phase modulators. Typical arrays can transition from one phase state to another in tens of nanoseconds. With appropriate light sources and optical phase discriminator systems, MEMS ribbon arrays may be used to project optical images.
MEMS ribbons' high speed enables linear (one-dimensional) arrays of ribbons to do the work of traditional spatial (two-dimensional) light modulators. Linear arrays create line images which may be scanned across a two-dimensional scene to ‘paint’ a two-dimensional image. Video frame rates of approximately 100 Hz to 1 kHz may be achieved in this way, fast enough to produce flicker-free video of complex visual scenes.
Linear arrays may also be used without scanning to create two-dimensional images, such as stripe patterns or bar codes, which vary along only one dimension. These simple images can be produced at frame rates as high as approximately 1 MHz or more. Depth capture systems based on observations of stripe patterns can take advantage of these high frame rates to enable advanced signal detection techniques.
FIG. 1 is a top view of part of a MEMS ribbon array 105. In FIG. 1, only 48 ribbons are shown (e.g. 110, 112, 114), but a typical array contains roughly a few hundred to roughly a few thousand ribbons. Coordinate axes are provided with FIG. 1 to facilitate comparison with FIGS. 2 and 3. Although ribbon dimensions may vary widely depending on particular applications, typical ribbons are roughly 100 to 300 microns long (y-direction), roughly 2 to 6 microns wide (x-direction), and roughly 0.1 to 0.3 microns thick (z-direction). Ribbons may be made from high-stress silicon nitride coated with aluminum or other materials to enhance optical reflectivity.
FIGS. 2A and 2B show a side view of a single MEMS ribbon at rest and under the influence of an applied voltage, respectively. In FIG. 2 ribbon 205 is supported by end-supports 210 over substrate 215. In FIG. 2A light ray 220 arrives at approximately normal incidence to ribbon 205 and is reflected as light ray 225. In FIG. 2A, ribbon 205 is at rest, not under the influence of external forces. In FIG. 2B, a voltage has been applied between ribbon 205 and substrate 215. The voltage pulls the ribbon from its rest position 230 toward the substrate by an amount, Δz, as shown in the figure. The optical phase, φ, of a light ray reflected from a ribbon depends on the displacement, Δz, according to:
      ϕ    -          ϕ      0        =      2    ⁢          (                        2          ⁢          π                λ            )        ⁢    Δ    ⁢                  ⁢          z      .      Here φ0 is the phase of a ray reflected from the ribbon when it is in its rest position and λ is the wavelength of light.
In conventional MEMS ribbon drivers, analog ribbon drive voltages are synthesized with high-precision digital-to-analog converters (DAC). A 12-bit DAC, for example, provides 4096 different drive voltage levels for a ribbon which leads to correspondingly fine control over the optical phase of light reflected from the ribbon.
When a MEMS ribbon array contains thousands of ribbons and each one is driven by its own precision DAC, the price and complexity of array drive electronics may become prohibitive. Furthermore precision DACs consume electrical power which is often in short supply in battery powered devices.
Therefore, what are needed are systems and methods for inducing analog MEMS ribbon movements from digital signals without using expensive, power-hungry, high-precision digital-to-analog converters.